Surface treatment of bright annealed strip



United States Patent 3,287,237 SURFACE TREATMENT OF BRIGHT ANNEALED STRIP Joseph W. Wilton, Wallingford, Conn., assignor'to Allegheny Ludlum Steel Corporation, Brackenridge, Pa.,

a corporation of Pennsylvania. A I

No Drawing. Filed Feb. 6, ,1964, Ser. No. 343,153

9 Claims. (Cl. 204-140) This application is a continuation-in-part of application Serial No. 183,026, filed March 28, 1962, now abandoned entitled, Surface Treatment of Bright Annealed Strip.

This invention relates to improvements in the surface quality of stainless steel and relates in particular to a method of producing stainless steel strip or sheet products that will exhibit superior corrosion resistance properties.

In the manufacture of stainless steel strip or sheet products, ingots, billets and slabs :are rolled at elevated temperatures (1600 F.2300 F.) to a gauge thickness of about .100" andare then cold roll-anneal cycled to finish gauge. The cold roll-anneal cycle consists of reducing the gauge of the steel by passing it through rollers which apply pressure to opposite sides of the plate or band so as to mechanically reduce the gauge. After several passes through such rolls,it is necessary to heat treat the steel to relieve the internal stresses and cold working effects of such mechanical deformation before further cold roll passes may be effected. In the manufacture of strip products the stress relieving heat treatments are generally effected by passing the strip through a furnace in a continuous manner. The furnace temperature and strip speed are gauged so that the strip will reach a temperature of about 1500" F. or more (depending on the type of stainless steel being processed). After eachanmeal or heat treatment (including hot rolling and any subsequent normalize or anneal), oxide scale must be removed prior to any subsequent'cold'rolling. Such oxide scale is normally removed by pickling or mechanical treatment. Strip products are continuously passed through solutions that chemically and/ or electrolytically dissolve and lift or raise the oxide scale from the surface of the steel. For example, such steelmay be electrolytically pickled in aqueous solutions of sulfuric, nitric or nitric-hydrofluoric acids for the purpose of removing oxide scale prior to further cold rolling or prior to shipping to a customer for his use. Cold rolling rolls are smooth and polished so that finished strip and sheet products are also smooth and polished. Since the final cold rolled product must also be annealed to overcome the effects of cold working, the finished product has a pickled finish which is duller than the as-cold rolled finish. However, for some grades, such as AISI Type 430 stainless steel, finishes may be effected that are relatively bright and attractive.

Cold rolled and annealed AISI Types 430 stainless steel (and modifications of this. grade including Types 433, 434, 435 and 436) are commonly used bythe automobileindustry for trim and grille applications. Although such steel is resistant to the corrosive effects of ordinary outdoor atmosphere, the corrosive environments caused by road deicing salts and cinders frequently cause undesirable rust bleeding and pitting of; such trim.

Other grades of stainlesssteel, such .asAISI .TypeslOl and 301, possess superior corrosionresistance when compared with the ferrite stainless steels, such as Type 430. Although these grades show better general. corrosion resistance than the fern'tic grades, they have also been found to be susceptible to rust bleeding and pitting when subjected to the corrosive environments experienced by automobile trim.

3,287,237 Patented Nov. 22, 1966 Lack of adequate corrosion resistance to withstand the corrosive environments of auto trim has been at least partially attributed to chromium depletion of the stainless steel surface during annealing and pickling. During heat treatment, chromium on'or near the surface of the steel matrix oxidizes preferentially to the iron and the loss of chromium from the surface results in a migration of the chromium content in the .area immediately adjacent the surface to the surface itself to oxidize and form the tenacious scale layer on the steel surface that is subsequently removed in a pickling bath. Since the chromium content of the steel 'is primarily responsible for its corrosion resistance, the chromium depleted surface is far more susceptible to corrosive attack' than-the balance of the matrix of the stainless steel. To eliminate the problems of chromium depletion and its resultant low general corrosion resistance, it is nowcommon practice to conduct stress relieving anneals, including continuous strip anneals, in a reducing or non-oxidizing atmosphere. The resultant stainless steel surface is both brighter-in appearance anddoes not suffer from chromium depletion because damaging oxide scales do not form, and as a result such steels exhibit improved general resistance to corrosion. However, it has been found that such bright annealed stock is susceptible to a pitting-type of corrosive attack that substantially reduces the attractiveness of bright annealed strip or sheet for the proposed automotive applications.

It has now been found that by employing the method of the present invention, which includes bright annealing, a stainless steel strip productmay be produced that possesses all the desirable attributes of bright annealed stainless steel but which exhibits superior anti-pitting characteristics.

It;is therefore the object of the present invention to provide a method to improve the anti-pitting characteristics of bright annealed stainless steel.

It is also the object of'the present invention to restore the anti-pitting characteristics of stainless-steel strip and sheet products which have been annealed in the presence of a reducing atmosphere.

Other objects and advantageous features will be obvious from the following description.

In general, the present invention relates to a method of enhancing the anti-pitting characteristics of stainless steelby first bright annealing the stainless steel and'then subjecting the bright annealed stock to anodic electrolytic treatment in 'anaqueous bath that contains an organic acid in solution.

The purpose of bright annealing is to avoid the dull pickled matte type finish obtained'when oxide scale is chemically removed from'the stainless steel surface, as well as to improve corrosion resistance. Stainless steel strip is passed through a shielded furnace enclosure that is conventionally a vertically elongated chamber which resembles a tower. The tower arrangement which is conventional and well known in the art of manufacturing stainless steel saves space and permits the strip to progress rapidly through the furnace while being withinthe heat zone for a sufficient time to permit'adequate annealing. Stainless steel is conventionally annealed within a temperature range of from about 1250"F. to 2050F., depending on the grade and the time at temperature. The atmosphere itself that is maintained within the furnace enclosure is generally reducing and is conventionally hydrogen or cracked ammonia, which is a combination of hydrogen and nitrogen. It has been found that the moisture content of such an atmosphere must be low to avoid some oxidation (preferably below a dew point of about 40 F.). The heat zone of the furnace is generally designed to heat the strip as it rises in the tower and cool while it descends so that it is not at a high temperature at additions recited above.

3 the exit end of the furnace and will not be subjected to oxidation at this point.

The term stainless steel, as applied in the present specification, is intended to include all the steels classified by the American Iron and Steel Institute as being standard grades of stainless steel. These include the Type 400 Series stainless steels'that contain chromium in amounts of from about by weight, to about 30%, by weight, and generally less than 1% carbon, such as AISI Types 410 and 430, and additionally the AISI Type 300 Series which contains, in addition to Cr and C, a nickel content of from 6 to 30% which renders the steel structure austenitic, such as AISI Types 301, 302 and 304, and the 200 Series steels which contain not only nickel in amounts of 1 to 10%, but also up to about 30% Mn. and .60% N as additional austenitizers. Such various stainless steel analyses may contain additionally, as impurities or alloy- 7 ing ingredients, small amounts of P, S, Cu, Mo, Se, B, Be, C0, W, Ti, Cb, .Ta, V, Zn, Al, Si, rare earths, etc. All stainless steels, however, contain chromium Within the range of from about 10% to 30% and carbon up to about 1%. The Cr content in every instance is the element that primarily affects the essential property of oxidation and corrosion resistance, and consequently the article of the present invention may be broadly said to be composed partly of a steel that consists essentially of carbon in an amount up to about 1%, chromium from 10 to 30%, nickel from 0 to 30%, manganese from 0 to 30%, and the balance iron.

Of the .400 Series of stainless steels, Type 430 and its modifications (small additions of one or more of Mo, Cb, Cu), are the grades most frequently produced in strip form for automotive (trim) applications. AISI Type 430 and its commercially available modifications (Types 433, 434, 435 and 436) are bright annealed at temperatures of from about 1400 F. to 1600? F. The 300 Series stainless steels, and particularly the grades specified above (Types 301, 302 and 304), are conventionally produced in strip form and may be annealed at temperatures of from about 1700 F. to 2100 F. Such annealing temperatures would include modifications of these compositions that include the impurities or additions mentioned above. The 200 Series compositions (AISI Types 201 and 202) are bright annealed within a temperature range of from 1600 F. to 2000 F. Such treatment also includes steel modifications containing small amounts of the impurities 'or Time at temperature for continuous bright annealing may be as short as a fraction of one second and as long as several hours (per inch of gauge), but is preferably about 60 minutes (per inch of gauge). Such treatment avoids formation of oxide scale, obviating any need for the type of pickling which results in a pickle matte finish and elfects a surface which is brighter than a No. 2 strip finish.

, Stainless steel is generally cold rolled from the hot rolled band in two stages, and is conventionally annealed at an intermediary gauge before the final cold rolling. Bright annealing may be effected at the intermediary gauge, but is generally carried on only as the last heat treatment. The strip'must be in the heat zone of thefurnace for a sufiicient time to come up to the desired temperature. For example, Type 430 strip passing continuously through a bright. annealing furnace which is at -a temperature of about 1875 F., is generally conducted through the furnace at speeds equal to about 60 minutes per inch of thickness.

The subsequent electrolytic treatment maybe conducted continuously as the stainless steel strip emerges from the annealing furnace, but may be conducted at any time after such treatment and before its use. For example, the electrolytic treatment of the present invention may be conducted after slitting or even a light skin pass type of cold rolling. In spite of the reducing atmosphere employed during continuous annealing, a slight amount of oxidizing can occur, particularly on the high manganesecontaining grades such as AISI Type 201 stainless steel, and may be so slight as to be not visually discernible.

Such insignificant oxidation does not interfere with the improved general corrosion resistance of the brighter strip 1 product, but must be removed. The conventional method 1 of eliminating such light scale is to treat the steel in an aqueous nitric or sulphuric acid bath in which process the steel being treated may or may not be made alternately cathodic and anodic, or some combination thereof.

In the method of the present invention, it may also be desired or required tofirst effect a cathodic'treatment before treating the steel anodically. Such cathodic treat- 1 ment may, however, be effected in the bath of the :present invention rather than by employing a separate nitric or sulfuric acid bath for such treatment.

The preferred cathodic treatment consists of rendering the steel strip or sheet cathodic for from .1 to 5 seconds at a current density of from about 0.05 to 1.0 amperes per square inch ofsurface area at a temperature of from about 40 to 120 F. This treatment may be rendered in 1 The strength of acid solution used may vary, and the acid solutions may be employed at strengths varying from 1%, by weight, of the acid to the saturation point of the 1 acid. Excellent results have been obtained by employing from about 3% to 36%, by weight, acetic acid in water,

or from 3% to 40%, by weight, formic acid in water. It

may be desirable to add electrolytes, such as sulphuric acid, or highly ionized salts such as sodium acetate, par.- ticularly' to the acetic acid bath, to improve the throwing power of the bath.

The following specific examples are given to illustrate the method of the present invention, but in no way limit the invention to the exact embodiments illustrated.

Coils of cold rolled stainless steel strip (about .025'. gauge) which had been hot rolled from stainless steel slabs and cold roll-anneal cycled were bright annealed by passing the strip continuously through bright annealing furnace at a speed of about60 seconds per inch of gauge. The atmosphere employed was a reducing atmosphere composed of cracked ammonia (25% nitrogen balance essentially hydrogen, dew point below about .40 F.). The temperature of the furnace varied in accordance with thetype of steel being annealed. For the AISI Type 430 and similar grades, the furnace temperature was about 1875 F., while for AISI Type 201 the furnace temperature was about 2000 F., and for AISI Types 301 and 305 the temperature was 2000" F. The exact analyses of the A modified grade of AISI Type 430 which contains some Mo and Cu. v

Panels sheared from someuofthe bright annealed coils (about 3" x 8".) were treatedelectrolytically in various acid-water solutions by immersing the samples in the solution while maintaining them anodic at a current density of about 0.1 ampere per square inch. Any scale or deposit appearing during bright annealing had first been removed by maintaining the steel cathodic in a 5%, by weight, water solution at about 0. 1 ampere per square inch for 2-3 minutes. The various solutions are set forth in Table II. The tests were conducted for times of 1, 5, and 30 seconds, and were conducted generally in three different strength solutions. The panels were then immersed in a 10%, by weight, ferric chloride solution at room temperature for minutes, after which the pitting frequency was rated as: 1=03 pits per flt-inch square of surface; 2=4-10 pits per A-inch square of surface, and 3-=over 10 pits per A-inch square. The various strength solutions employed are set forth below the electrolyte or acid employed, and these figures represent the weight percent of acid employed, the balance being water. The three figures appearing under Results represent the results obtained by the three strength solutions employed; where only one strength solution was used, only one pitting result is reported. All tests and treatments were conducted at approximately room temperature.

Another series of tests was carried out on seven additional heats of AISI Type 430 stainless steel, two heats of AISI Type 201 stainless steel and one heat each of A181 Types 301 and 305 stainless steels. These samples were all sheared from strip which had been bright annealed as reported for the tests of Table l]. The panels (3" x 8") were then treated electrolytically in aqueous solutions indicated in Table 111 below with the specimen anodic (0.1 ampere). The panels were treated for a total time of about 10 seconds. All tests were conducted at approximately room temperature. All test panels after electrolytic treatment were immersed in a 10%, by weight, ferric chloride in water solution at room temperature for 15 minutes. The panels were then withdrawn and evaluated for pits in the same manner as for Table H. Results were as follows:

Panels of bright annealed Type 434 stainless steel strip (AISI Type 430 plus about .50% M0) were electrolytically treated in an aqueous formic acid solution (specimens anodic about 0.1 ampere, about 10 seconds), the

formic acid solution bein g'about 30%,b'y weight, formic acid, balance water. These samples were then treated in ferric chloride solution (as above). The results were substantially identical with the acetic acid treated samples as reported in Table HI above.

Also, test specimens of Type 434 stainless steel we're electrolytically treated in 10%, by volume, aqueous solutions of each of citric acid, monochloracetic acid, oxalic acid, tartaric acid and picric acid, with the specimens being maintained anodic for about 30 seconds at a current density of 0.2 amp per square inch, and in saturated aqueous solutions of each of benzoic acid and boric acid, with the specimens being maintained anodic for about 30 seconds at current densities of 0.1 amp per square inch, and in saturated aqueous solutions of stearic acid and pyrogallic acid, with the samples being maintained anodic for 30 seconds with the current density being less than 0.1 amp per square inch. Each of these samples, together with an untreated, bright annealed sample, was immersed in a 10%, by weight, ferric chloride solution for two hours, and the pits per square inch were rated. Each of those samples treated in the solutions of benzoic acid, boric acid, citric acid, monochloracetic acid, tartaric acid and picric acid had substantially fewer pits per square inch than the untreated sample, whereas those samples treated in the stearic acid and pyrogallic acid solutions had approximately the same number of pits per square inch as the untreated sample.

From the results reported above, it can be seen that electrolytic treatment of the bright annealed stainless steel in organic acid solutions in which the acid is sufficiently soluble to provide a concentration which will pass at least 0.1 amp per square inch materially improves the anti-pitting characteristics of the steel surface over the as-bright annealed material. It may also be seen that similar electrolytic treatment in nitric or sulphuric acids does not result in a corresponding improvement.

The data of Table III tends to confirm that of Table H in showing an improvement in resistance to pitting for the acetic solution. The double set of results reported for the acetic acid tests is the result of testing two samples in each instance.

I claim:

1. The method of providing a bright stainless steel surface which is resistant to corrosive pitting which stainless steel has been bright annealed by heating in the presence of a reducing atmosphere which method comprises, immersing said steel in an aqueous solution consisting essentially of an organic acid selected from the. group consisting of acetic, formic, citric, monochloracetic and picric, said acid being of suflicient concentration to pass a current density of at least 0.1 ampere per square inch of surface area of immersed steel, maintaining said solution at a temperature within the range between 40 F. and F., and rendering said steel anodic at a current density of from 0.1 to 1.0 ampere per square inch of surface area of immersed steel.

2. The method of claim 1 wherein said acid is acetic acid.

3. The method of claim 2 wherein the aqueous solution contains from 3 to 36%, by weight of acetic acid.

4. The method of claim 1 wherein the acid is formic acid.

5. The method of claim 4 wherein the aqueous solution contains from 3 to 40%, by weight, formic acid.

6. The method of claim 1 wherein the acid is citric acid.

7. The method of claim 1 wherein the acid is monochloracetic acid.

8. The method of claim 1 wherein the acid is picric acid.

9. The method of claim 1 wherein the steel is rendered anodic from about 0.5 to about 60 seconds.

(References on following page) 7 8 References Cited by the Examiner 3,097,980 9/1963 Swan et a1. 148- 135 3,247,086 4/1966 Goldstein et a1 204'140 UNITED STATES PATENTS FOREIGN PATENTS 2531;; 20PM; 5 1,027,550 2/1953 France. gg r 204140'5 JOHN H. MACK, Primary Examiner. Anderson 204140.5 R. MIHALEK, Assistant Examiner. 

1. THE METHOD OF PROVIDING A BRIGHT STAINLESS STEEL SURFACE WHICH IS RESISTANT TO CORROSIVE PITTING WHICH STAINLESS SETTL HAS BEEN BRIGHT ANNEALED BY HEATING IN THE PRESENCE OF A REDUCING ATMOSPHERE WHICH METHOD COMPRISES, IMMERSING SAID STEEL IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF AN ORGANIC ACID SELECTED FROM THE GROUP CONSISTING OF ACETIC, FORMIC, CITRIC, MONOCHLORACETIC AND PICRIC, SAID ACID BEING OF SUFFICIENT CONCENTRATION TO PASS A CURRENT DENSITY OF AT LEAST 0.1 AMPERE PER SQUARE INCH OF SURFACE AREA OF IMMERSED STEEL, MAINTAINING SAID SOLUTION AT A TEMPERATURE WITHIN THE RANGE BETWEEN 40*F. AND 120*F., AND RENDERING SAID STEEL ANODIC AT A CURRENT DENSITY OF FROM 0.1 TO 1.0 AMPERE PER SQUARE INCH OF SURFACE AREA OF IMMERSED STEEL. 